1. Field of the Invention
This invention relates to a metal gasket held between opposed surface of a pair of structural bodies, for example, a cylinder block and a cylinder head of an engine so as to seal the same surfaces.
2. Description of the Prior Art
A metal gasket has heretofore been used by being interposed between opposed surfaces of structural bodies, which constitute an engine, such as a cylinder block and a cylinder head of a multicylinder engine, and tightening the cylinder head against the cylinder block with tightening bolts so as to seal the same surfaces. The metal gasket is disposed so that a combustion gas does not leak from contacting portions of these opposed surfaces, and various types of metal gaskets are used for this purpose.
A high-performance engine for automobiles which is designed so as to have lighter weight, smaller dimensions or a higher output has recently been developed, in which a cylinder head comprising an aluminum alloy is employed for the purpose or reducing the weight. Although the aluminum alloy has light weight, it has a low rigidity. Consequently, the temperature variation occurring in accordance with the starting and stopping of the engine or the operational condition thereof causes the thermal expansion or contraction of the cylinder head to increase, or stress, i.e. a thermal load imparted to the gasket due to repeated stress occurring during the use of the engine and the condition of a cold heat cycle during the same time causes the distortion of the gasket to increase. As a result, the breakage of the gasket and a rapid decrease in a sealing surface pressure occur, and this causes the leakage of a gas from a narrow clearance between the opposed surfaces to occur in some cases.
Under the circumstances, a method of using a metal gasket as an effective seal means which can be substituted for a conventional gasket of asbestos has come to be generally known, and various types of metal gaskets have been developed.
A metal gasket is provided with bore holes opposed to cylinder bores formed in a cylinder block so that pistons are moved reciprocatingly therein, and also annular beads around the circumferences of the bore holes. Such beads are also formed in the vicinity of the circumferences of through holes, such as bolt holes which are other than the bore holes. When the metal gasket is fixed by being tightened by bolts with the gasket held between a cylinder head and a cylinder block, contacting portions, which are elastic with respect to fixing surfaces, of the beads, especially, the portions of the opposed surfaces which are around the circumferences of the cylinder bores form annular contacting sections, whereby the portions of the fixing surfaces which are around various holes, such as combustion chambers, water holes, oil holes and bolt holes are sealed.
A metal gasket usually comprises a plurality of metal plates. The metal plates include an elastic metal plate provided with beads thereon, an intermediate plate and a regulating plate. These metal plates are fixed to one another by a means, such as welding at the portions thereof which are in the vicinity of the bore holes so as to enable the metal plates to be transported, stored and assembled as a metal gasket, one of engine-forming parts. In such a conventional fixed structure, it is necessary to employ large-scale welding equipment, which constitutes one of the causes of preventing the cost of manufacturing the metal gasket from being further reduced.
The metal gasket having a function of a stopper and disclosed in Japanese Patent Laid-Open No. 347065/1992 is formed by laminating partial shim members on the portions of a bead-recess-carrying side of an elastic metal plate, on which beads are provided around bore holes thereof, which start at the edge portions of the bore holes, and fixing the shim members to the flat portions, which are closer to the bore holes, of the elastic metal plate. According to this metal gasket, even when the portions of the beads which contact the shim members are fully compressed, the remaining portions are not. The remaining portions have a restoring force, so that they can display the function of stoppers even when the distance between adjacent bore holes is short. When two beaded elastic metal plates are used, they are laminated on each other so that the bead ridges face in the opposite directions, i.e. outward directions, and partial shim members extending from the edge portions of the bore holes are provided between the bead-recess-carrying sides of the two elastic metal plates.
The metal gasket disclosed in Japanese Patent Laid-Open No. 175579/1992 is capable of regulating the thickness thereof while maintaining at a uniform level the balance of stress occurring in each base plate, and designed to secure a high sealability thereof. In the metal gasket, two elastic metal plates having bore holes correspondingly to combustion chambers in a cylinder block, and beads around the circumferences of the bore holes are laminated on each other so that the bead ridges face in the opposite, i.e. outward directions, and thickness increasing members are welded between the portions of the base plates which are on the side of the bore holes.
The open deck type engines include a full-open deck type engine in which each cylinder body is formed independently with a water jacket formed therearound, and a semi-open deck type engine in which some or all of cylinder bodies are combined with one another into a unitary structure, around which a water jacket is provided.
The open deck type engines include an engine having a cylinder block of an aluminum alloy, and a cylinder liner formed of special cast iron by taking the abrasion resistance thereof into consideration. In the cylinder block provided with a dry cylinder liner not exposed directly to the water jacket, a boundary line between the cylinder liner and a cylinder casing in which the cylinder liner is fitted appears on the portion of a deck surface which is around a cylinder bore. When a metal gasket in which metal plates are laminated with the bead ridges facing in the outward directions is interposed between opposed surfaces of a cylinder head and a cylinder block of this engine, the ridges of the beads on an elastic metal plate is positioned on the boundary line on the deck surface in some cases. Since the cylinder casing and cylinder are separate members, a slight height difference is liable to occur on the boundary line, so that there is the possibility that, when the cylinder head is tightened against the cylinder block, a sealed region becomes unreliable.
If the shim members which give a thickness regulating function to a bead-carrying elastic metal plate of a metal gasket in the open deck type engine can be provided with sealability improving functions, such as an irregularity offsetting function and a surface pressure distributing function with respect to the whole of an annular deck positioned between cylinder bores and a water jacket, it will produce preferable results in view of the necessity of providing parts with complex functions. In an engine having a dry liner-carrying cylinder block, a boundary line exists between the dry liner and a cylinder casing. Therefore, it demands a structure capable of reliably sealing the portions of a deck surface which are around the cylinder bores, in which portions height irregularity is liable to occur. The cylinder block having a dry linear is formed out of an aluminum alloy in some cases, so that protecting the deck surface of an aluminum alloy, the rigidity of which is lower than that of cast iron, against a high sealing surface pressure is also demanded.